Wool is classified primarily by fiber diameter, measured in microns. Three grading systems are used in the United States: the American Blood Grade System, the Spinning Count (Bradford) System, and the Micron System. Of these, the micron system is the international standard and the one preferred by wool buyers and manufacturers. Beyond diameter, wool is also evaluated by staple length, crimp, tensile strength, and cleanliness before it reaches the market.
The Three Grading Systems
The American Blood Grade System is the oldest of the three, developed in the early 1800s. The grades originally referred to how much Merino genetics a sheep carried, since Merino sheep produce the finest wool. A “1/2 Blood” sheep was roughly half Merino. Today the system has nothing to do with actual breeding. It simply describes a broad range of fiber diameter, from “Fine” at the top to “Braid” at the coarsest end. The full scale runs: Fine, 1/2 Blood, 3/8 Blood, 1/4 Blood, Low 1/4 Blood, Common, and Braid.
The Spinning Count System (also called the Bradford or English system) is more precise. Each grade number represents how many 560-yard hanks of yarn could theoretically be spun from one pound of clean wool. A grade of 80s means 80 hanks, while a grade of 40s means only 40. Finer wool contains more individual fibers per pound, so it yields more yarn. The higher the spinning count, the finer the wool.
The Micron System measures average fiber diameter directly, in microns (one micron equals 1/25,400 of an inch). This is the most precise method and the one used in international trade. A Merino fleece might measure 18 microns, while a coarse carpet wool could exceed 40 microns. Because it relies on actual measurement rather than subjective judgment, the micron system has become the global standard.
Micron Ranges by Grade
The U.S. Department of Agriculture defines specific micron ranges for each wool grade. Fine wools sit at the lower end of the scale. Grade 80s wool averages 17.70 to 19.14 microns, grade 70s runs from 19.15 to 20.59, and grade 64s spans 20.60 to 22.04. These fine grades correspond to the “Fine” and “1/2 Blood” categories in the Blood system and are what you’ll find in high-end suiting and next-to-skin garments.
Medium wools cover a wide middle range. Grade 62s (22.05 to 23.49 microns) through grade 48s (31.00 to 32.69 microns) includes the bulk of wool used for blankets, outerwear, and general textiles. At the coarse end, grade 40s wool measures 36.20 to 38.09 microns, grade 36s runs 38.10 to 40.20, and anything above 40.21 microns falls into the coarsest category, typically used for rugs and carpets.
Staple Length and Processing Class
Within each fineness grade, wool is further classified by staple length, which determines how it can be processed. “Combing” wool has longer fibers and higher spinning quality. “Clothing” wool has shorter fibers and less spinning value. A label like “58s combing” tells a buyer both the fineness (58s grade) and that the fiber is long enough for worsted yarn production.
Minimum length requirements vary by grade. Fine wool needs a staple length of at least 3 inches. Half Blood requires 3.25 inches, 3/8 Blood needs 3.50 inches, and 1/4 Blood and coarser grades require at least 4 inches. Anything shorter than these thresholds is difficult to process at the mill and loses significant value.
Crimp, Strength, and Condition
Crimp is the natural waviness of wool fibers, and it serves as a visual indicator of fineness. Finer wools have more crimps per inch, appearing tightly curled, while coarser wools look wavy or nearly straight. Tighter crimp also means more surface area, which catches more dirt and holds more lanolin. That’s why finer fleeces generally have lower clean yields: a greater percentage of their weight washes away during processing.
Tensile strength matters because weak wool creates waste. Classers test strength by gripping both ends of a lock and pulling it apart. If the fibers snap, the wool is classified as “tender.” A more serious defect is a “definite break,” where every lock in the fleece snaps cleanly at the same point. This typically results from a period of illness or poor nutrition that weakened fiber growth at a specific time. Wool with a definite break is placed in the lowest category regardless of its other qualities, because those short, broken fibers fall out during manufacturing and become waste.
Condition also plays a role. Wool that appears dry and dull lacks the luster of freshly shorn fleece and can feel harsh to the touch, which may lead evaluators to perceive it as a lower grade than its actual fiber diameter would suggest.
How Fiber Diameter Is Measured
The traditional method of measuring fiber diameter uses a projection microscope, which magnifies short fiber sections 500 times onto a screen where they can be measured individually. This method is still referenced as a baseline, but two newer instruments have largely replaced it in commercial testing.
The Laserscan and OFDA (Optical Fibre Diameter Analyser) instruments have been in use since 1995 under international testing standards. Both are faster and more consistent than the projection microscope. In international round trials, Laserscan showed the lowest variation between laboratories, with a 95% confidence limit of plus or minus 0.05 microns for 20-micron wool. OFDA was nearly as precise at plus or minus 0.06 microns. The two instruments agree with each other more closely than either agrees with older methods, with differences between them rarely exceeding 0.3 microns.
Breed Categories
Sheep breeds fall into broad wool categories that align with the grading systems. Fine-wool breeds like the Merino and Rambouillet produce fibers under roughly 22 microns and command the highest prices. Medium-wool breeds, which include many of the common meat-and-wool dual-purpose breeds, produce fibers in the middle range. Long-wool breeds like the Lincoln, Romney, and Border Leicester grow fibers with exceptional staple length and luster. Their wool is especially popular with handspinners and fiber artists. At the coarsest end, carpet-wool breeds produce thick, durable fibers suited to rugs and heavy textiles rather than clothing.
Yield and Vegetable Matter
Raw wool straight off the sheep (called “greasy wool”) contains lanolin, sweat, dirt, and plant material that all wash away during processing. The percentage of usable fiber left after scouring is the wool’s “yield.” If clean wool sells for $3.50 per pound and the yield is 57%, the greasy price works out to about $2.00 per pound. Buyers care deeply about yield because it determines how much finished product they actually get from each bale.
Vegetable matter, including seeds, burrs, and grass, is measured during testing and reported on sale certificates. Higher contamination levels lower the wool’s value because removing plant material adds processing steps and cost.
International Trade Certification
In the global wool market, most bales are sold with a test certificate issued under standards maintained by the International Wool Textile Organisation (IWTO). Core samples are pulled from each bale in a sale lot, then tested for fiber diameter, vegetable matter content, staple strength, length, and color. The resulting IWTO certificate travels with the wool through the entire supply chain, from auction to processing mill.
Testing laboratories that issue IWTO certificates must conform to international standards and are audited annually. The testing specifications themselves are updated each year in the IWTO’s Red Book and White Book, which serve as the industry’s reference documents. This system gives buyers worldwide a consistent, reliable way to compare wool from different countries and producers.
Sustainability Certification
Beyond physical grading, wool can also be classified by how it was produced. The Responsible Wool Standard (RWS), managed by Textile Exchange, certifies wool from farms that meet requirements for animal welfare, land management, and social welfare. Certified farms must respect the Five Freedoms of animal welfare, use progressive methods to protect soil health and biodiversity, and maintain safe working conditions. When a product carries the RWS label, it verifies that the wool inside can be traced back to farms meeting those criteria.